Organic semiconductors typically exhibit a rich variety of ordered phases. Orientation occurs during processing, often by passing through lyotropic liquid crystalline phases, and dried films exhibit orientation at many length scales. Liquid crystallinity appears almost ubiquitous in organic semiconductors we have studied, including behaviors similar to nematics, smectics, or sanidics. Uniaxial orientation where the conjugated plane exhibits a preferential “face on” or “edge on” orientation is the most commonly characterized aspect of orientational order, and mature techniques such as near edge X-ray absorption fine structure (NEXAFS) spectroscopy and grazing incidence X-ray diffraction (GIXD) are frequently employed to measure it. Significant challenges remain, even for this seemingly simple aspect of orientational order. For example, these approaches are often found to be insufficient when films exhibit a complex depth profile of orientation, composition, or both.<br/><br/>As the materials diversity of organic semiconductors increases thanks to achievements in synthetic chemistry, and as the complexity of formulation, blending, and coating increases as process design knowledge matures, there is a commensurate need for more advanced approaches to characterizing orientational order. Biaxial orientation is increasingly observed, where “face on” or “edge on” preferences are combined with an in-plane preference imparted by a substrate or processing vector. Domain-relative orientations are also been observed, with correlations in domain-to-domain orientation thought to influence charge transport.<br/><br/>I will describe some emerging approaches to measure orientational order in organic semiconductors. Resonant soft X-ray reflectivity (RSoXR), a method capable of measuring thin film depth profiles using the spectroscopic principles of NEXAFS, will be discussed. I will also summarize progress on in-situ spectroscopic and scattering-based structure measurement techniques that are combined with ex-situ probes including imaging and image analysis to determine how orientation originates during the processing of organic semiconductors. These new measurements will support science-based approaches for orientation control and optimization, helping to maintain the rapid pace of organic semiconductor technology development in an era where increasing complexity renders matrix-style empirical approaches too inefficient to contemplate.

Emerging approaches to measure orientational order in organic semiconductors

256th National Meeting (2018)

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